Power supply switching circuits are employed in many different applications, e.g. where the interruption of power from a particular power supply is a foreseeable event and an alternative source of power must be switched into operation in its place. Prior art switching circuits exist which are assembled from discrete components. Such circuits may be interposed between a load and the power sources, or they may themselves include an auxiliary power source. Although these circuits provide effective and power-efficient solutions to the problem, they generally employ bipolar junction transistors which cannot be integrated with CMOS processes. Additionally, such circuits generally employ capacitors and other components that are relatively difficult to integrate.
Real time clocks are among a variety of loads to which it is desirable to provide continuous power. Such clocks typically comprise power-consuming oscillators and memories. They may be used simply to provide data for a display of current time, or they may be employed to provide a timing mechanism for initiating various functions in different kinds of environments. Such clocks are required to remain in continuous operation, even when the system in which the clock is employed is not operating, so that the clock's normal power source may be turned off.
In the prior art as well as in the present invention, low power batteries provide a power source to such clocks when regular system power is off. This arrangement supplies to only those components which are required to track current time and thus power consumption is minimized. However, many prior art circuits permit a small current flow from regular system power to the batteries. Such a charging feature is undesirable when battery power is provided by certain types of long-life batteries, e.g. lithium batteries.
Real time clocks of the type described typically consist of integrated circuits. Ideally, the power supply switching circuit (though not the power supplies themselves) is located on the same chip as the load, e.g. the clock serviced by the power supply. Also, the switching circuit should be of minimum size. Current carrying elements in the circuit should be both of relatively low resistance and minimum size. Among the advantages of such a monolithic, integrated arrangement versus a system arrangement, are savings of space on the chip, reduced cost and improved quality control in the manufacturing process. However, for the reasons discussed above and because space on an integrated chip is typically at a premium, it has not been possible to date to integrate these components, except at excessive cost.